1. Field of the Invention
The present invention relates to a focusing method and lens system for maintaining a constant transverse magnification in a lens system comprising a front lens group and a rear lens group by moving said anterior lens group and posterior lens group in accordance with changes in the object plane (position of the object).
2. Description of the Related Art
As shown in FIG. 2A, a lens system comprising a first lens group 1 having a focal length equivalent to an object distance f.sub.1, and a second lens group 2 having a focal length f.sub.2 is considered. The object image on object point 3 is formed at image point 4.
As shown in FIG. 2B, when, for example, the object point 3 is moved a distance [a] only to the lens side, an a focal construction is formed between the first lens group 1 and the second lens group 2. If the first lens group 1 is moved a distance [a] only to the image side, the transverse magnification .beta.(.beta.=-f.sub.2 /f.sub.1) is nonvariable. That is, in the present instance, the moving distance of the first lens group 1 is large, and disadvantageously, the diameter of the moving group also becomes larger.